Radiolysis influence on low alloy steel atmospheric corrosion at 80°C

International audienceThe degradation of iron-based materials by atmospheric corrosion is a well-known problem thatmay have incidence, especially for the storage of radioactive nuclear wastes. It is of importanceto evaluate the damages due to this kind of corrosion within long time-periods. One parameterwhich can influence atmospheric corrosion in the case of storage container is irradiation. Asatmospheric corrosion happens when a water film condensates at the surface of a material,radiolysis of this water film in contact with air can have for consequence the formation of acid anoxidizing species which can modify the corrosion process.As a consequence the aim of this work is to evaluate the influence of γ-irradiation onatmospheric corrosion of low alloy steel. In this goal, an experiment, called CASIMIR, has beendeveloped. During this experiment, samples of low alloy steel have been introduced into smallstainless steel container. In these ones, two different media has been introduced. The firstmedium is a small quantity of water in order to fix a relative humidity (RH) of 100 %. Thesecond medium is a melt of KH$_2$PO$_4$/ K$_2$HPO$_4$in a small quantity of water corresponding to arelative humidity of about 70% at the surface of the samples. The small containers have beenintroduced in a heating system which enables to fix a temperature of 80°C. This system has beeninstalled in an irradiation pool with three $^{60}$Co sources which fix, according to the location of thestainless steel in the system, a dose rate of 80 Gy.h$^{-1}$ or 20 Gy.h$^{-1}$ at the surface of the low alloysteel samples.After six, nine and twelve months of corrosion in these conditions some samples were extractedof the experiment and analyzed by different methods. Gravimetry after desquamation of thecorrosion layer enables to measure total corroded thickness in function of RH, dose rate andtime. As show on figure 1, after 6 month of exposure, sample aged with a RH of 100% are morecorroded than the one aged at 70%. Moreover the dose rate does not have an effect on thecorroded thickness for 20 Gy.h$^{-1}$ whereas it increases corrosion when the dose rate is equal to 80Gy.h$^{-1}$. This result is confirmed for longer times. Gas analysis after corrosion experiment haveshown that for the higher corrosion rate, oxygen was totally consumed after 6 month and revealalso a production of hydrogen. Microscopical observations also confirm the influence of doserate on atmospheric corrosion.All the results show that dose rate does not influence atmospheric corrosion until a valuebetween 20 and 80 Gy.h$^{-1}$. For a dose rate up to these thresholds, irradiation increase corrosionrate on low alloy steel in presence of high values of relative humilities.6th International Works